Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (hereinafter also referred to as MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
Normally, a hydrogen gas produced from hydrocarbon based raw fuel by a reformer is used as a fuel gas supplied to the fuel cell. In the reformer, in general, a reformed gas (fuel gas) is produced, e.g., by applying partial oxidation reforming or steam reforming to such hydrocarbon based raw fuel, e.g., fossil fuel such as metal or LNG.
In this case, since the partial oxidation reformer induces exothermic reaction, the reaction can be started at relatively low temperature, and thus the start-up performance and the follow up performance are good. However, the reforming efficiency is poor.
In contrast, the steam reformer has good reforming efficiency. However, since the steam reformer induces endothermic reaction, the start-up performance and the follow up performance are poor at relatively low temperature.
In this regard, for example, a method of operating a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2007-179756 (hereinafter referred to as Conventional Technique 1) is known. The operating method includes the steps of mixing a plurality of types of reforming gases including hydrocarbon gas, reforming the plurality of types of reforming gases mixed in the mixing step by use of reforming catalyst to produce a hydrogen-containing gas, and generating electricity by a solid oxide fuel cell by use of the hydrogen-containing gas obtained in the reforming step.
It is monitored whether the mixing step, the reforming step or the power generation step is performed under predetermined heat-balancing conditions or not. If the mixing step, the reforming step or the power generation step is performed under the predetermined heat-balancing conditions, then water vapor is mixed with a hydrocarbon gas in the mixing step, and thereafter steam reforming is performed in the reforming step. If the mixing step, the reforming step or the power generation step is not performed under the predetermined heat-balancing conditions, then an oxygen-containing gas is mixed with the hydrocarbon gas in the mixing step, and thereafter partial oxidation reforming is performed in the reforming step.
In the operating method according to the disclosure, if the heat balance is changed from the optimum state, reforming is switched from steam reforming to partial oxidation reforming for making it possible to perform the reforming without water vapor and prevent carbon deposition. Further, since partial oxidation reforming is an exothermic reaction, the temperature which has been lowered can be raised again, and it contributes to restoration of the heat-balancing to the optimum state. Thus, if the optimum state of heat-balancing is not satisfied due to various factors during operation of the fuel cell, and the heat energy is insufficient for the reforming step, then reforming is switched to partial oxidation reforming thereby to compensate for the inefficient heat energy, and thus adjustment thereof is achieved. Further, by switching of the reforming method, large variation in the heat balance can be prevented, and instability in the output of power generation of the fuel cell is eliminated.
Further, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2010-238595 (hereinafter referred to as Conventional Technique 2) is known. The fuel cell system includes a reformer for performing steam reforming of a fuel gas supplied to a fuel cell, water supply means for supplying water to the reformer, and control means for controlling the water supply means.
The water supply means includes a water tank for storing water to be supplied to the reformer, a pump for supplying the water stored in the water tank to the reformer under pressure, flow rate detection means for detecting the amount of water supplied to the reformer by the pump, and pressure detection means for detecting the pressure of the fuel gas in the reformer. At least at the time of starting operation, the control means switches operation of the pump from the suspension state to the pressurized water supply state to start supply of the water under pressure. Thereafter, the control means performs a water supply determination process for determining whether the water is supplied to the reformer or not based on the detection result by the pressure detection means.
In Conventional Technique 2, operation of the pump is switched from the suspension state to the pressurized water supply state to start supply of the water to the reformer under pressure. Thereafter, water supply determination process for determining whether the water is supplied to the reformer or not is performed based on the detection result by the pressure detection means. By the water supply determination process, since it is possible to determine whether the water is supplied to the reformer or not based on the detection value of the pressure detection means regardless of the detection value of the flow rate detection means, it is possible to determine whether the water is supplied to the reformer or not regardless of the amount of the supplied water.